I routinely travel with two FlexPens and just leave them in their Frio in a baggie in my carry-on. Dozen + trips through SEA and DFW, never been asked about it. Now I don't have a syringe, just a pen needle.

Couple times when I've been travelling with a dozen bottles of a gel medication (in their sealed boxes with Rx labels), my carryon has been pulled and eye-balled. Nothing else.

I wouldn't bother doing anything special. If they pull your bag and ask, say that it's medically necessary (key phrase) and if they push back, politely ask for a supervisor.